Intercom systems are used in a variety of commercial contexts to provide communication between two or more individuals located remote from one another. Although even simple intercom systems are capable of providing private point-to-point communication between just two individuals, state-of-the-art systems are designed to provide "conference-type" communication, connecting several intercom stations with one another simultaneously, so that several individuals can talk jointly with one another. Examples of commercial contexts utilizing such systems include television production facilities, stadiums, theaters, and the like.
Conventionally, intercom systems used by such establishments employ a matrix of switches, called a crosspoint switch. A crosspoint switch is utilized when a system has many sources and many destinations, any of which may need to communicate with a changing mix of any of the others at various times during system operation. Even if one particular facility can "hard-wire" its sources and destinations in a static configuration, any system intended for use by facilities with differing needs can benefit from the ease of configurability that a crosspoint switch provides. In short, a crosspoint switch is employed any time the desired connectivity between a system's multiple sources and destinations cannot be known in advance of the system's hardware design.
To make use of a crosspoint switch, the system architecture is set up as follows: instead of connecting the various source equipment and destination equipment directly in point-to-point fashion, each source and destination is connected only to the crosspoint; which makes cross connections between the sources and destinations internally. A signal thus makes two hops in going from source to destination; it must always travel via the crosspoint.
A crosspoint switch can be implemented as a matrix of SPST switches between input and output lines, as shown in FIG. 1. Typically the switches are tiny semiconductor switches which are digitally controlled via software. The signal path through the switches may either be analog or serial digital. In FIG. 1 a 4.times.4 crosspoint switch matrix is shown. Any of the source signals, A through D, can be connected to any of the destination paths, E through H. If, for example, it were desired to route source signal C through to destination F and at the same time route source signal B through to destination H, one would close switches 9 and 7, as shown in FIG. 1.
Input signals can also be sent out to multiple destinations in parallel, as shown in FIG. 2 (i.e., a "Y" connection). But one may not route more than one signal onto a single destination path, as shown in FIG. 3. This attempt causes a direct short between the outputs of two driver amplifiers elsewhere in the system, and could actually result in electrical damage either to the switches or to the external signal source equipment. (The correct way to combine two or more signals would be to route the source signals to separate outputs, and then sum them--external to the crosspoint--with a mixer unit).
The disadvantage of the matrix-of-switches approach to implementing a crosspoint is that the cost of the matrix grows in geometric proportion to the number of inputs and outputs to be connected. That is, though a 4.times.4 matrix only requires 16 switches, a 100.times.100 matrix requires 10,000 switches, and a 500.times.500 matrix would require 250,000 switches. Thus, beyond a certain system size, a hierarchical matrix of matrices must be constructed in order to achieve complete cross-connectivity. In addition, implementing analog "Y" connections to multiple destinations in parallel will result in signal degradation due to impedance mismatches if too many destinations are driven. To prevent this, input and output drivers must be included, further adding to the crosspoint's cost while degrading its signal-to-noise ratio. Thus, in practice, the matrix-of-switches crosspoint is practical only for small to medium size systems.